Number of southern right whales, Eubalaena australis, and population trend in the neighbourhood of Península Valdés during the period 1999-2011 by means of aerial and boat surveys

CRESPO, ENRIQUE A; PEDRAZA, SUSANA N; DANS, SILVANA L; COSCARELLA, MARIANO; SVENDSEN, GUILLERMO M; DEGRATI, MARIANA

Buenos Aires

Workshop; IWC SOUTHERN RIGHT WHALE ASSESSMENT WORKSHOP; 2011

International Whaling Commission

Payne and colleagues carried out long term studies in Southern right whales since the early 70´s and estimated population size and other parameters derived from sighting data based on individual pattern of callosities. The rate of increase r was estimated close to 8%. We developed a method for monitoring the population, which could lead to study seasonal changes within and through years. Whales were classified as: a) mother-calf pairs (MC), b) solitary individuals (SI), c) breeding groups (BG) considering one female and n-1 males. Twelve aerial surveys were performed between May 1999-December 2000 and 16 between October 2004-December 2006, flying parallel to the coastline at a distance of 500-1000m and an altitude of 500 feet, from the mouth of Chubut River (42°30´) to Puerto Lobos (42°) totalling a coastal strip of 350 nm. Around 95% of the whales are within the strip and the number of whales in the strip can be considered a measure of relative abundance. The period between flights ranged from 40-50 days which is beyond the average permanence of whales in the area, so ideally during each census new individuals were being counted. Every year a bell shaped curve was obtained increasing in May and declining in December when no whales remain in the area. Maximum number of whales is counted always during September and was as follows: 556 in 1999, 566 in 2000, 732 in 2005, 798 in 2006. The rate of increase for the period 1999-2006 was estimated from the slope of the linear regression of the log-number of whales through time. Despite there is a short period of time between the series of flights, the rate of increase was estimated to be 5.1% (CI=3.4-6.8), lower than that estimated by mark-recapture methods but with confidence intervals in the same order of magnitude.